Adhesive technology is advancing day by day. Advanced adhesives include an acrylic emulsion adhesive. This acrylic emulsion adhesive is an adhesive in which an acrylic copolymer as a tack component is dispersed in particulate form in water. No organic solvent is used in the acrylic emulsion adhesive, so that the working environment is free from organic solvent pollution and is also free from the danger of inflammation. Therefore, the acrylic emulsion adhesive is being widely used.
This acrylic emulsion adhesive is commonly produced by dispersing an acrylic monomer in water with the use of an emulsifier and polymerizing the acrylic monomer. A tackifying component is generally added to the acrylic emulsion adhesive in order to improve the adhesion performance thereof. For example, a polymerized rosin ester, a terpene resin or a petroleum resin is used as the tackifying component. It is extremely difficult to disperse such a tackifying component in water, so a very large amount of emulsifier is needed to uniformly disperse the tackifying component in water. The emulsifier is a surfactant, and, the higher the emulsifier content, the lower the water resistance of the obtained acrylic emulsion adhesive. On the other hand, when the amount of added emulsifier is decreased for ensuring water resistance, uniform dispersion of the tackifying component becomes difficult, so that a new problem arises in that the adhesion properties are different between parts where the tackifying component is present and parts where the tackifying component is absent.
In this situation, there have been proposed processes in which a tackifier resin is present in the reaction system of emulsion polymerization of an acrylic monomer for producing an acrylic polymer in place of the conventional addition of the tackifying component after the production of the acrylic polymer.
For example, Japanese Patent Laid-open Publication No. 59(1984)-58069 discloses an invention of a strongly adherent emulsion-type pressure sensitive adhesive comprising a mixture of:
100 parts by weight, in terms of solid contents, of an tackifier resin emulsion (A) obtained by conducting a water-based emulsion polymerization of a resin solution consisting of 100 parts by weight of a tackifier resin dissolved in 30 to 150 parts by weight of an .alpha., .beta.-monoolefin monomer; and
20 to 1000 parts by weight, in terms of solid contents, of an .alpha., .beta.-monoolefin polymer emulsion (B) which contains no tackifier resin and exhibits a polymer second transition temperature of not higher than 10.degree. C.
As is apparent from the Examples described in the above publication, the tackifier resin emulsion (A) is produced by dispersing an alkyl acrylate such as butyl acrylate and acrylic acid as acrylic monomers and a tackifier resin in water with the use of an anionic surfactant and a nonionic surfactant and performing a polymerization. The .alpha., .beta.-monoolefin polymer emulsion (B) is produced in the presence of an anionic surfactant and a nonionic surfactant in the same manner except that no tackifier resin is used. The obtained tackifier resin emulsion (A) and .alpha.-monoolefin polymer emulsion (B) are blended together to thereby obtain the strongly adherent emulsion-type pressure sensitive adhesive. Therefore, this strongly adherent emulsion-type pressure sensitive adhesive contains a relatively large amount of surfactant which has been used as the emulsifier. Thus, in the polymerization of the above acrylic monomer together with the tackifier resin in the presence of a catalyst, the use of a relatively large amount of emulsifier is inevitable for stable emulsification of the acrylate monomer containing the acrylic acid monomer.
The strongly adherent emulsion-type pressure sensitive adhesive described in the above publication is directly used as a pressure sensitive adhesive or an adhesive, so that the used emulsifier still remains in the strongly adherent emulsion-type pressure sensitive adhesive.